Although hardness and resistance to wear are the most commonly known industrial applications of the unique properties of diamond, it is likely that future applications will utilize the electronic and optical properties of this material. The rarity of the natural gemstone indicates that the supply from this source is grossly insufficient to match the demand from the polishing, cutting and drilling industries. Furthermore, supplies of natural diamond are inadequate for the burgeoning semiconductor industry, which is expected to occupy 60% of the worldwide sales for diamond (projected at $985 million) by 1996.
The term "diamond-like carbon" (DLC) is generally applied to films of carbon that have properties that resemble, but do not match, those of true diamond. For many applications, however, the distinction is not important, and the ability to produce such coatings at substrate temperatures well below that needed for true diamond makes DLC useful for protective coatings and in some optical applications. These films are generally amorphous, though they may contain very small crystallites less than about 20 nm in size. DLC usually contains a level of hydrogen up to 50 atomic percent as a consequence of prior art chemical vapour deposition processes which use hydrocarbon/hydrogen gas mixtures.
Following an extensive investigation of the carbon phase diagram in the 1940s, the successful synthesis of artificial diamond was achieved in 1953 at ASEA in Sweden and in 1954 at General Electric in the United States. This method required both the high temperatures and high pressures which are characteristic of the natural process, but additionally included a metal solvent and catalyst to permit growth at reasonable rates. A disadvantage of this prior art process is that while the metallic impurity level resulting from such a catalyst is not significant from the point of view of mechanical applications, it is unacceptable for the optical and electronic applications which now represent the basis of the modern market expansion.
A second high pressure method to synthesize diamond was discovered in 1961, and was industrially developed by Du Pont. This method harnessed the high pressures in explosively-driven shock waves to initiate a direct conversion of graphite to diamond.
Both of the above-discussed prior art methods are capable of producing synthetic diamond grit for abrasive use at economical cost, although the inclusion of residual graphite, metals and carbides makes such products unsuitable for "gem quality" applications. Such synthetic forms are available (General Electric, Sumimoto Electric) with masses up to 2 carats (about 0.4 gram), or up to several millimetres in size, at about $150 per piece, although the majority do not find end-use in the jewelry market, but as heat sinks in the electronics industry. This rather surprising property of diamond is illustrated by the fact that in its purest form (only the isotope 12 present) diamond may exhibit a thermal conductivity up to ten times that of copper. Diamond-coated heat sinks are thus an attractive alternative, as are semiconductor devices based on diamond; in comparison to silicon, for operation at extreme temperatures wherein the switching of large power in small volumes is feasible.
In addition to the above methods of forming diamond by high pressure synthesis, several groups have investigated the preparation of low pressure (metastable) carbon forms, making use of chemical procedures to restrict the rate of conversion to the stable (graphitic) phase. For example, U.S. Pat. No. 3,030,188 (Eversole) and a paper by Derjaguin (Growth of Diamond and Graphite from the Gas Phase, in Russian) both report the formation of diamond from the gas phase decomposition of carbon-containing species such as carbon monoxide or hydrocarbons such as methane and acetylene. In both of these prior art processes, a "seed" crystal was used, on which both diamond and graphite were deposited. The process was interrupted to remove the undesired graphitic phase by preferential etching of the graphite, by heating in atmospheres of either hydrogen or oxygen. The deposition and etching cycles continued until a measurable mass had been accumulated.
The requirement for a suitable etchant continues in present research (e.g. usually atomic hydrogen is used), but the first to achieve deposition in a continuous cycle was J. C. Angus, who used a methane/hydrogen mixture at deposition rates low enough to prevent stable graphitic nuclei from forming, as described in U.S. Pat. No. 3,630,677. It was also demonstrated by Angus that boron doping could produce semiconducting diamond films.
By 1976, Spitzyn, Bouliov and Derjaguin demonstrated unmistakeable evidence of both large-faceted diamonds and continuous films, not only on diamond substrates, but on other materials as well (see B. V. Spitsyn, L. L. Bouiliov, B. V. Derjaguin, J. Cryst. Growth 52 (1981) 219. English version of above work published in a Russian language journal in 1976). The methods all involved a "super equilibrium" with diamond, either by catalysis, by a hot filament, or by an electrical discharge plasma, techniques which form the basis of present day methods albeit with modification in experimental parameters. For example, by the early 1980s, a group at NRIM in Japan deposited diamond films using a hot filament to produce dissociation of hydrogen in a methane-hydrogen mixture (see S. Matsumoto, Y. Sato, M. Kamo, N. Setaka, Jpn. J. Appl. Phys. 21 (1982) L183; M. Kamo, Y. Sato, S. Matsumoto, N. Setaka, J. Cryst. Growth 62 (1983) 642; and S. Matsumoto, J. Mat. Sci. Lett. 4 (1985) 600). Others have continued with alternative sources of atomic hydrogen such as radio-frequency and microwave-frequency plasmas. These methods are currently known as chemical vapour deposition (CVD), under the main categories of plasma-assisted CVD, thermally assisted (or "hot wire") CVD and reactive vapour deposition. The latter can be implemented simply using an oxyacetylene flame, or using a mix of halocarbon gas and hydrogen. Variations exist in which a bias voltage is applied between the hot filament and substrate to accelerate or repel charged gas phase species. In extreme cases, a glow discharge or direct-current plasma arc may be employed.
It has been discovered that the very rapid energy input available using modern pulsed lasers (typically up to 100 MW peak power) can potentially produce novel materials due to the non-equilibrium conditions created in the ablation of a source material. Additionally, there exists the possibility of photochemical processing of the resultant film. At the present time, lasers have been used principally as an alternative evaporative or thermal source of film material.
For example, in 1983 Fedoseev and Derjaguin reported a novel method of diamond synthesis which involved the exposure of a flowing loose powder of carbon black, in air, to a continuous wave carbon dioxide laser flux (see D. V. Fedoseev, V. L. Bukhovets, I. G. Varshavskaya, A. V. Lavrent'ev, B. V. Derjaguin, Carbon 21 (1983) 237; and D. V. Fedoseev, I. G. Varshavskaya, A. V. Lavrent'ev, B. V. Derjaguin, Powder Technology 44 (1985) 125). This simple treatment appeared to transform some fine carbon black particles to various high pressure phases, including diamond. Separation of the high pressure phases from the untransformed carbon black was achieved by physical and chemical methods, and diamond formation was verified by means of electron and X-ray diffraction. The mechanism for the transformation is not understood, although the authors speculated that the transformation of carbon black to diamond occurs because of the rapid heating and cooling of the particles. Since thermal equilibrium cannot occur on such a short timescale, it is presumed that the massive thermal stress occurring at high temperature results in the transformation, and that rapid cooling is similarly required to maintain the existence of such non-equilibrium products. Other materials have been studied by this technique, such as high pressure polymorphs of silica.
The use of laser ablation as a source of carbon vapour is known, although it is not as widespread as CVD methods. It may be used for the thermal evaporation of most elemental materials, technically important compounds such as high temperature super-conducting alloys, and others including cubic boron nitride. For example, pulsed laser ablation is the only technique known to yield heteroepitaxial boron nitride films on silicon. Direct laser etching of patterns in thin metallic and polymeric films has also been contemplated as an alternative to ion milling or photoresist masks. Laser etching has also been used for the trimming of passive components.
Both Cuomo and Collins have claimed the production of diamond and DLC films by means of the laser ablation of carbon in combination with modest or very high electrical bias to accelerate ionized species in the plasma plume to the substrate. (See (1) J. J. Cuomo, D. L. Pappas, J. Bruley, J. P. Doyle, K. L. Saenger, IBM Research Division, T. J. Watson Research Centre. Paper presented at Int. Conf. on Metallurgical Coatings and Thin Films, San Diego, April 1991. U.S. patent applied for.; and (2) R. A. Collins, Rice University, Texas. Paper presented at Diamond Film '90, Crans Montana, Switzerland, September 1990, published in Surface and Coatings Technology, June 1991 (in press). U.S. patent issued 1990).
The inventors in the present application have also investigated a variation of the above method, in which either a buffer (e.g. helium) or reactive gas (eg. hydrogen) is introduced at appropriate pressure between the ablation plume and the substrate. (See E. B. D. Bourdon, W. W. Duley, A. P. Jones, R. H. Prince, Surface and Coatings Technology, 47 (1991) 509.; E. B. D. Bourdon, R. H. Prince, Applied Surface Science 48/49 (1991) 50. By this means, the partial pressure of carbon is independent of the buffer gas mixture, hence the technique may be considered "additive" in the manner in which hydrogen is introduced, in contrast to CVD processes in which the carbon/hydrogen ratio is always limited by the feedstock hydrocarbon gas. Good quality clear DLC films have been produced by this approach, with optical gaps up to 3.5 eV (transmission to about 400 nm wavelengths). A distinct advantage over plasma-assisted CVD, for example, is the ability to deposit at low (e.g. room) temperatures on substrates such as plastics that would not survive the higher temperatures used in most CVD methods.
A recent development by Narayan et al is reported in J. Narayan, Science, March 1991. This method involves the alleged production of diamond crystals by laser irradiation of copper, previously implanted with high energy carbon ions. The method makes use of the very low solubility of carbon in copper, and the role of the laser is purely thermal, (i.e. for heating the carbon to high temperatures at high atomic density in the copper "host" matrix). Despite the close match in lattice constants between diamond and several transition metals and alloys, growth on such metals has been unsuccessful, presumably because of the high solubility and mobility of carbon in such cases. Copper appears to be a notable exception.